The detection and interdiction of illicitly trafficked Special Nuclear Material (SNM) is very important in the ongoing anti-terrorist activities undertaken by homeland security agencies. U.S. Patent Application No. 2005/0105665 by Lee Grodzins and Peter Rothschild for a system of detection of neutrons and sources of radioactive material, published May 19, 2005, provides the following state of technology information: “There is a need to find sources of radiation and other nuclear material that are clandestinely transported across national boundaries. The sources of clandestine nuclear material may be in the form of “dirty bombs” (e.g., a conventional explosive combined with radioactive nuclides designed to spread radioactive contamination upon detonation), fissile material, and other neutron and radiation emitting sources that may present a hazard to the public. During recent years, the United States government has placed mobile vehicles at strategic areas with gamma ray detectors dedicated to the task of finding fissile material. Atomic explosives may be made from 235U, a rare, naturally occurring, isotope of uranium that lives almost 109 years, or 239Pu, a reactor-made isotope that lives more than 104 years. 235U decays with the emission of gamma ray photons (also referred to as ‘gammas’), principally at 185.6 keV and 205.3 keV. 239Pu emits a number of gamma rays when it decays, the principal ones being at 375 keV and 413.7 keV. These gamma rays are unique signatures for the respective isotopes. But fissile material invariably contains other radioactive isotopes besides those essential for nuclear explosives. For example, weapons grade uranium may contain as little as 20% 235U; the rest of the uranium consists of other isotopes. The other uranium and plutonium isotopes reveal their presence by gamma rays emitted by their daughters. For example, a daughter of 238U emits a high energy gamma ray at 1,001 keV; a daughter of 232U, an isotope present in fissile material made in the former USSR, emits a very penetrating gamma ray at 2,614 keV; and a daughter of 241Pu emits gamma rays of 662.4 keV and 722.5 keV.”
U.S. Pat. No. 4,201,912 issued May 6, 1980 to Michael L. Evans et al. and assigned to the United States of America as represented by the U.S. Department of Energy, provides the following state of technology information: “A device for detecting fissionable material such as uranium in low concentrations by interrogating with photoneutrons at energy levels below 500 keV, and typically about 26 keV. Induced fast neutrons having energies above 500 keV by the interrogated fissionable material are detected by a liquid scintillator or recoil proportional counter, which is sensitive to the induced fast neutrons. Since the induced fast neutrons are proportional to the concentration of fissionable material, detection of induced fast neutrons indicates concentration of the fissionable material.”
U.S. Pat. No. 3,456,113 issued Jul. 15, 1969 to G. Robert Keepin and assigned to the United States of America as represented by the U.S. Atomic Energy Commission, provides the following state of technology information: “An apparatus and method of detecting, identifying and quantitatively analyzing the individual isotopes in unknown mixtures of fissionable materials. A neutron source irradiates the unknown mixture and the kinetic behavior of the delayed neutron activity from the system is analyzed with a neutron detector and time analyzer. From the known delayed neutron response of the individual fission species it is possible to determine the composition of the unknown mixture. Analysis of the kinetic response may be accomplished by a simple on-line computer enabling direct readout of isotopic assay.”
Traditional neutron detectors that have been used to augment gamma-ray detection systems typically rely on “gross-counting” to detect an increased neutron presence that may provide an indication of elevated fission from an unknown source. Fissile material detection with passive neutron multiplicity counters use the observation of correlated neutrons to indicate presence of fissile sources as opposed to industrial neutron sources (e.g., AmLi or AmBe). When measuring uranium, the spontaneous fission rate that is produced requires a passive measurement on the order of one day to see a correlated fission signal. To speed this process, an external neutron source or generator may be used to induce fission in the U235. Long-lived neutron sources like AmLi may induce fission and work well when the detection mechanism uses a Poisson discrimination technique as described herein. For enhanced operator safety, electrically generated neutron sources, commonly available in the form of DD (deuterium-deuterium) or DT (deuterium-tritium) sources are used. These generators are neutron source devices that contain compact linear accelerators and that produce neutrons by fusing isotopes of hydrogen together. The fusion reactions take place in these devices by accelerating the deuterium, tritium, or a mixture of these two isotopes into a metal hydride target, which also contains deuterium and/or tritium.
These electric neutron sources are intended to be either pulsed or steady state in neutron production. Electric neutron sources are regularly used to induce fission, where the user irradiates a sample and then looks for delayed neutrons as a signal that fissile material is present. However, such an approach is relatively hazardous and inefficient because of the high neutron intensity necessary to induce enough residual delayed fission product activity. A more efficient alternative to observing the delayed neutron fraction is to look for neutrons produced while the interrogation beam is on. This is more efficient because the induced fission rate is controlled by the fission cross section, unlike the delay-based method which delivers a few percent of this induced fission. This requires the detector to distinguish between the electric source neutrons and the induced fission neutrons, which has traditionally been impractical with present portable DD or DT electric neutron sources.
What is needed, therefore, is a method of distinguishing between these two types of detected neutrons by recognizing patterns of neutrons created and/or counted by the system.
Another disadvantage associated with present systems is that the electric generators used in such systems are almost always made using AC rectifying, high voltage DC power supplies. This introduces the problematic effect of electrical ripple on the DC supply that can cause correlation of neutron product, thus introducing unwanted correlation in the DD or DT neutron generator.
What is further needed, therefore, is a Poisson neutron source for use in in-beam interrogation systems that imposes virtually no ripple to distort the correlation of generated neutrons in a neutron detection system.